1. Field of the Invention
The present invention relates to a solventless melt-shapeable thermally curable heat conductive dielectric polymer material, and more particularly relates to a heat dissipation substrate containing the same.
2. Description of the Related Art
In recent years, white LEDs have become a very popular new product attracting widespread attention all over the world. Because white LEDs offer the advantages of small size, low power consumption, long life, and quick response speed, the problems of conventional incandescent bulbs can be solved. Therefore, the applications of LEDs in backlight sources of displays, mini-projectors, illumination, and car lamp sources are becoming increasingly important in the market.
Although LEDs represent the future of illumination applications, many problems still remain unsolved. For example, with a high power LED for illumination, only about 15% to 20% of input power is converted into light, and the remaining 80% to 85% is converted into heat. If the heat is not dissipated to the environment efficiently, the temperature of the LED die becomes too high, thus influencing the light emitting intensity and service life of the LED die. Therefore, the heat management of LED devices is very important to performance and durability.
FIG. 1 shows a known heat dissipation substrate 10 for an electronic device such as an LED device (not shown). The heat dissipation substrate 10 includes a heat conductive dielectric material layer 12 and two metal foils 11 formed on upper and lower surfaces of the heat conductive dielectric material layer 12. The electronic device is placed on the upper metal foil 11. According to a known process for the heat dissipation substrate 10, a heat conductive filler (such as aluminum oxide grains) is added to liquid epoxy resin with various amount of solvents, and such mixture is blended with a curing agent to form a slurry. Then, the gas, which results from the evaporation of the solvent, in the slurry is removed by vacuum, and the slurry is daubed on the lower metal foil 11. The upper metal foil 11 is placed on the slurry to form a composite structure of metal foil/slurry/metal foil. The composite structure is then hot-pressed and cured to form the heat dissipation substrate 10. The resulting hot-pressed and cured slurry forms the heat conductive dielectric material layer 12.
However, the known process is restricted to the property of the slurry and has the following drawbacks: (1) The known process has to be finished within a certain time period; otherwise the slurry will cure and cannot be daubed on the metal foil, and the slurry is wasted. (2) While hot-pressing, some slurry overflows out of the metal foils 11. (3) When a temperature is reached during hot-pressing, separation of solid and fluid may occur, causing the conductive filler to be unevenly distributed in the heat conductive dielectric material layer 12 which reduces heat dissipation efficiency. (4) The storage of the slurry is not easy and sticky slurry limits the flexibility of the manufacturing process of the heat dissipation substrate. For example, the heat dissipation substrates with various shapes cannot be made efficiently, (5) There are some residual solvent in the dielectric layer to cause the evaporation of the solvent, formation of gas bubbles, and the interfacial delamination during the consecutive high temperature solder reflow process.
A traditional heat conductive circuit board can be manufactured by mixing liquid epoxy resin, solvents, a heat conductive filler and a curing agent to form a slurry, which is then coated on metal foil and heated to reach the B-stage before being hot-pressed to form a printed circuit board. Alternatively, a traditional FR-4 circuit board can also be manufactured by solution coating epoxy resin on a glass fiber cloth and heating the epoxy resin to B-stage, and then the glass fiber cloth is hot-pressed to form a glass fiber printed circuit board.
The above-mentioned conventional methods need low-viscosity slurry containing a lot of solvents; however, the solid filler portion and the liquid polymer solution portion may easily separate in a low-viscosity slurry due to the higher density of the conductive filler than that of the surrounding liquid matrix. The filler may naturally settle toward the bottom layer, causing non-uniform mixing and solid-liquid separation issue that affects heat dissipation efficiency. Furthermore, the slurry cannot be easily stored due to the solid-liquid separation problem as well as the shelf life issue.
Attempts were made to impregnate glass fiber cloth with the low viscosity slurry which includes thermal conductive fillers, solvents, and thermalset polymers. Due to the low thermal conductivity of glass fiber (about 0.36 W/mK), the heat conductive dielectric polymer material prepared from impregnated glass fiber usually has poor heat dissipation capability.
In summary, the low-viscosity slurry needed for known heat conductive circuit boards causes a problem in that the solid-liquid separation can easily occur. In addition, due to the low thermal conductivity of glass fiber, the printed circuit board made of glass fiber has poor heat dissipation. Thus, a new solventless melt-shapeable thermally curable heat conductive dielectric material serving as a high heat dissipation interface of a circuit board is required.